Mysql cumulative total only working sometimes

I'm using the following SQL to calculate the running total by date of a data processing function:

set @running_total := 0;
select date(event_date), (@running_total := @running_total + count(distinct de.iddocument)) AS cumulative_sum , count(distinct de.iddocument)
from document_event as de
left join document as d on d.iddocument = de.iddocument
where d.iddatastream = 142
and de.event_type = 'RESEARCHED'
and de.update_by_id is not null
group by date(event_date);

This SQL has been working perfectly for a year, however on the latest batch of data it no longer calculates the running total, it only displays each days total.